Plasma amplifiers



CALL-ii A. V. HAEF'F PLASIIA AMPLIFIERS 'seir. 17, 1957 Filed July 6", 1954 3 Shuts-Shut 1 A M r wanna A410,

3 Shuts-Shut 2 1 A. V. HAEFF' PLASM AMPLIFIERS Sept. 17, 1957 Filed h1g6. 1954 Arr/W1. lmaw K 440/,

' [fill/"IX p 1957 A. v. HAEFF 2,806,974

PLASMA AMPLIFIERS Filed Juiy e. 1952 3 Shuts-Shoot 3 H" k s i s 3 5 N Q \i I Q i Q Ira/11x United States Patent PLASMA AMPLIFIERS Andrew V. Haefl, Pacific Palisades, Calif asslgnor to Hughes Aircraft poration of Delaware Application July 6, 1954, Sel'iIl No. 441,520

9 Claims. (Cl. 3153.6)

This invention relates to wave-type amplifier devices and more particularly to wave-type plasma amplifier tubes. Patent No. 2,740,917, entitled Electron Stream Amplifier Tube, of Andrew V. Haeff, Serial No. 282,000, filed April 12, 1952 disclosed a wave-type tube wherein amplification of a space-charge wave propagated by an electron stream was effected by directing the electron stream through a medium having certain optimum resistance characteristics. In this application the ele'ctron stream obviously could not pass through a solid medium for any distance and consequently was made to simulate traversing a medium having the desired characteristics provided by a solid material by directing the electron stream contiguously along a surface of the material. Amplification of the space-charge wave was then produced by interaction of the stream electrons with electric fields generated by currents induced in the resistive medium by the space-charge wave.

According to the present invention, amplification of a space-charge wave propagated by an electron stream is produced by directing the electron stream through a. plasma region having a resistivity provided by ionized gas. In this device, amplification is effected in a manner similar to that disclosed in the aforementioned Haefi application in that the stream electrons constructively interact with electric fields generated by currents induced in the ionized medium by the space'charge wave. In the present device, however, the conductivity of the medium can be adjusted to the value at which optimum gain is obtained merely by controlling the degree of ionization or the pressure of the gas. The electron stream may either be separated from the plasma region by a thin glass wall, or alternatively, may be actually directed through the region. In the former case the ionization may be controlled independently of the electron stream whereas in the latter case the pressure of the gas in the plasma region is controlled to give adequate focusing of the electron stream due to the formation of positive ions along its path. Thus, a solenoid is not required for focusing the electron stream in the case where the stream is directed through the ionized gas. An outstanding advantage of this type of tube over conventional travelingwave amplifier tubes is that 'the space-charge can only be propagated in the direction of electron flow by the stream thus eliminating the possibility of the tube breaking into self-oscillation.

An additional embodiment of the present invention is also disclosed wherein the electron stream propagating a ,space-charge wave representative of a signal to be amplified is directed through a first plasma region which is separated by a thin glass wall from a second plasma region. In its operation, stream electrons constructively interact with electric fields produced by the currents induced by the space-charge wave in the first plasma region. These electric fields'in the first plasma region are, in turn, amplified" due to constructive interaction with electric fields in. the second plasma region generated by currents induced in this region by the electric fields in Company, Culver City, Calif., a cor- 2,806,974 Patented Sept. 17, 1957 the first region. These amplified electric fields in the first plasma region then interact to a greater extent with the space-charge wave propagated by.the electron stream to, in turn, increase the amount that it is amplified.

It is therefore an object of this invention to provide high-gain broad band amplification of electromagnetic waves, particularly in the microwave range.

A related object of this invention is to provide a method utilizing at least one plasma region for amplifying micro wave signals over a broad band of frequencies.

Another object of this invention is to utilize a plasma region both for amplifying a space-charge wave propagated by an electron stream and for focusing the electron stream along its path.

The novel features which are believed to be characteristic of the invention, both as to' its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. 1 shows an embodiment of the. invention in schematic form wherein the electron stream is separated from a plasma region by a thin glass wall;

Fig. 2 shows an additional embodiment of the invention in schematic form wherein the electron stream is directed through a first plasma region which is separated from a second plasma region by a thin glass wall; and

Figs. 3, 4 and 5 are schematic representations of embodiments of the invention wherein different means of controlling the ionization within the plasma region are employed.

'Referring'now to the drawings, Fig. 1 illustrates an embodiment of the invention comprising an envelope 10 having an enlarged portion 12 at the left extremity, as viewed in the drawing, for housing an electron gun 14. Electron gun 14 develops an electron stream which is directed along a path that occupies a cylindrical region concentric with the longitudinal axis of envelope 10. Envelope 10 is expanded to a cross sectional area several times that require for the electron stream to form an en larged cylindrical section 16 along a substantial length of the path in order to provide an enclosure for a plasma chamber 17 along the path of the electron stream. The envelope 10 is expanded further at each extremity of section 16 to form toroidal sections 18 and 20 for housing electrodes 22 and 24, respectively, to enable the gas in plasma chamber 17 to be ionized. A thin-walled glass tubular element 26 is disposed so as to extend contiguously along the path of the electron stream coextensive with plasma section 16 and is sealed to extensions 28 and 30 of envelope 10 to enable the chamber occupied by the electron stream to be evacuated independently of the plasma chamber 17.

A solenoid 29 for directing the electron stream along the path on the longitudinal axis of envelope 10 is posi tioned symmetrically about the length of envelope10. An appropriate direct-current is maintained in solenoid 29 by means of suitable connections to a battery 31 so as to produce a magnetic field which may be of the order of 500 gauss running axially along the length of the tube. The purpose of this magnetic field is to keep the electron stream focused or constrained for the entire length of the path. I

Electron gun 14, which developsthe electron stream, includes a cathode 32 with a heating element 34,11 focusing electrode 36 and an accelerating anode '38., element 34 is maintained at an appropriate temperature b means of a connection across a battery 40. One side oi heating elem eat 34 is connected to cathode 32 which is, in turn, maintained at a potential of the order of l000 volts with respect to ground by a connection to the negative terminal of; battery 42, the pos tive terminal of which is connected to ground. Focusing electrode 36 is of the Pierce type and is maintained at zero volts relative to cathode 32 by means of a suitable connection thereto. Accelerating anode 38 is at a potential of the order of 1000 volts positive with respect to the potential of cathode 32 by means of connection to ground.

Disposed concentrically about the path of the electron stream in the direction of electron flow are the matching ferrule 44 connected over a lead 46 to an input helix 48, tubular element 26, and an output helix 50 connected over a lead 52 to a matching ferrule 54. At the end of the path, the stream electrons are intercepted collected by collector electrode 56. Matching terminations 57 and 59 for input and output helices 48 and 50, respectively, are provided in the form of resistive coatings disposed on the outside of extensions 28 and 30 about the last several turns of the nearest ends of the respective helices 48 and 50. The purpose of these matching terminations 57 and 59 is to terminate electromagnetic waves propagated by the helices.

Tubular element 26, as previously mentioned, is sealed to the extensions 28 and 30 of envelope so that the chamber for the electron stream maybe evacuated independently of the plasma chamber 17. In addition to this, it is necessary that a space-charge wave propagated by the electron stream induce currents within the plasma chamber 17 to generate electric fields to interact with the electrons of the stream. To accomplish this the walls of tubular element 26 are made as thin as practicable, for example, of the order of .001". Further, in order to prevet a negative charge from accumulating on the tubular element 26, a highly resistive conductive coating 58 is disposed on its inner surface and extends over the shoulders at each extremity so that electrical contact may be made with input and output helices 48 and 50 and thus avoid detrimental wall charging effects which may otherwise be caused by the flow of the stream electrons. Thus, inasmuch as matching ferrules 44 and 54, input and output helices 46 and 50, and resistive coating 58 of tubular element 26 are all interconnected, they may be maintained at ground potential by a connection from matching ferrule 44 to ground, as shown.

As previously mentioned, collector electrode 56 is disposed at the end of the path of the stream to intercept and collect the electrons. In order to suppress secondary electron emission from collector electrode 56, its potential is maintained at a value of the order of 200 volts positive with respect to ground. This is accomplished by a connection to a battery 60 which has its negative terminal referenced to ground.

The radio-frequency input to the tube of the present invention is provided by an input waveguide 62 which forms an enclosure about the envelope 10 coextensive with lead 46 connecting matching ferrule 44 to input helix 48 in such a manner that the lead 46 is disposed symmetrically within the waveguide across its shortest dimension at a distance approximately equal to one-quarter wavelength at the operating frequency of the tube from a shorted termination 64. A virtual short plane is produced on the inner surface of waveguide 62 nearest electron gun 14 by means of a sleeve 66 which extends from the waveguide concentrically about matching ferrule 44 for a distance of one-quarter wavelength. An output from the tube is provided in the same manner by an output waveguide 68 having a shorted termination 78 and a sleeve 72 disposed about the lead 52 and matching ferrule 54 in the same manner as for the input.

In the manufacture of the tube, the plasma chamber 81 is filled with inert gas such as, for example, helium at a pressure of the order of 100 microns. Disposed within the two toroidal sections 18 and 20 at each extremity of the plasma chamber 17 are the ionizing electrodes 22 and 24, respectively. A potential is applied across these electrodes 22 and 24 so as to produce a plasma of the desired degree of ionization within the chamber 17. The actual magnitude of this potential may be of the order of several hundred volts and will depend, of course, upon the pressureandtypeofgasused. Thispotentialisappliedbya connection from electrode 22 to one extremity of a potentiometer 76 which is connected across a battery 78 and has an intermediate point connected to ground. An adjustable tap 80 of potentiometer 76 is then connected to electrode 24. The voltage across electrodes 22 and 24 is adjusted to a value that provides for optimum amplification of the tube.

In its operation, a microwave signal to be amplified is introduced through input waveguide 62 to the input helix 48. The electron stream interacts with this input signal to produce a growing wave on the helix 48 and, at the same time, increase the modulation of the stream. The signal at the end of helix 48 constitutes an electromagnetic portion propagated by the helix and a space-charge portion propagated by the electron stream. The electromagnetic portion of this wave is terminated by the resistive coating 57 while the space-charge wave is propagated on through the tubular element 26 surrounded by the plasma chamber 17. The plasma frequency in chamber 17 is adjusted so that the space-charge wave interacts with electric fields generated by currents induced in the ionized medium by the space-charge wave in such a manner that the space-charge wave increases in amplitude. In that the space-charge wave is representative of the microwave signal to be amplified, this amplification constitutes the gain provided by the tube. The walls of tubular element 26 are made as thin as possible to effect maximum interaction between the stream electrons and the electric fields in the plasma chamber 17 and thus produce maximum amplification.

As the electron stream, propagating this amplified space-charge wave enters output helix 5. an electromagnetic wave is induced thereon. The electrons of the stream interact with this electromagnetic wave induced on the helix 50 to cause it to increase in amplitude. The velocity of the electron stream is such that as the wave progresses through the helix, substantially all of the signal energy is transferred from the electron stream to the electromagnetic wave. At the end of the helix 5. the amplified electromagnetic wave is transferred to the output waveguide 68 and the remaining energy of the electron stream is dissipated at the collector 56.

An alternate embodiment of the tube of the present invention employing two plasma regions is illustrated in Fig. 2. In this embodiment an inert gas such as, for ex ample, helium, is introduced in the evacuated chamber occupied by the electron stream to provide a first plasma region, as A pressure is used in this chamber to electuate the production of a suficient number of positive ions by the stream electrons so as to neutralize the spacecharge forces within the stream to the extent that adequate focusing is provided for the stream. In the event that helium is employed in this chamber, this pressure is of the order of 1 micron depending on the stream current. Thus, with adequate focusing provided by the plasma region, u,, the focusing solenoid 28, along with its concomitant alignment and direct-current energin'ng requirements, is no longer necessary.

A second plasma region, 0,, is provided in the samemannerasinthetubeofFig. lexceptthatthe ionizing potential in this embodiment is applied between the matching terminations $7 and. of input and output helices 48 and 58, respectively, thereby making ionizs l t d sfl dfl a therwiththeirassoeiated toroidalaections18amunnecessary Also,inorder tomaintainauniformpoteutialgradiemalongtheentire length of thegeccnd plasma region, a resistive coating 82 is dispmedonthe inner surface of envelope 10 within the plasma chamber 17 and back along the extensions 28 and 30 to matching terminations 57 and 59, respectively, so as to make electrical contact therewith. Thus,

theapplication of the ioniz'ng potential to matching terminations 57 and 59 produces a uniform potential gradient along the complete length of the region occupied by plasma w, In order to effectuate this result, matching termination 57 is connected to ground and matching termination 59 is connected to a tap 84 of a potentiometer 86 which is connected across a battery 88, the negative terminal of which is connected to ground. The remaining elements together with their associated circuitry are the same as in the embodiment shown in Fig. 1.

In operation of the plasma amplifier tube shown in Fig. 2, the electron stream is modulated with a microwave signal to be amplified in the same manner as before. During this modulating process, the signal wave along input helix 48 constitutes a composite wave having an electromagnetic portion propagated by the helix 48 and space-charge portion propagated by the electron stream. At the termination of input helix 48, the electromagnetic portion of the wave is terminated by matching termination 57 and the space-charge wave propagated by the electron stream continues on through the region of plasma o This space-charge wave, is before, induces currents in the ionized medium which currents generate electric fields which interact with the electrons of the stream to cause the space-charge wave to grow. The electric fields generated in region of plasma 1a,, however, induce currents in the region of plasma at, to generate electric fields which increase the amplitude of the currents induced in region of plasma a, to further increase the rate at which the space-charge wave increases in magnitude.

After passing through the amplifying section of the tube, the signal energy constituting the space-charge wave propagated by the stream is transferred to the output waveguide section 68 by the output helix 50 in the same manner as before.

Alternative embodiments of the present invention wherein the electron stream is directed through a region of plasma to effectuate focusing of the stream and amplification of a space-charge wave propagated by the stream are shown in Figs. 3, 4 and 5 which illustrate different means of controlling the ionization of the plasma region.

More particulary, Figs. 3, 4 and 5 show embodiments that are similar to that of Fig. 2 except that only a single plasma region is used. Thus, the tubular element 26 separating plasma region at, from plasma region w, of the embodiment of Fig. 2 is not necessary in these alternative embodiments.

Referring specifically to Fig. B'there is illustrated an embodiment of the present invention wherein a uniform potential gradient is maintained along the length of the plasma region w, in the same manner as was done in the embodiment of Fig. 2. In this tube, the plasma region' w, is filled with an inert gas at a pressure suitable for focusing the electron stream. This pressure is a function of the stream current, the stream velocity, and the type of gas and may generally be of the order of 1 micron. The remaining elements and associated circuits are the same as described in connection with Fig. 2.

Fig. 4, on the other hand, shows a plurality of annular rings 90 spaced uniformly along and concentrically about the path of the electron stream. These rings are alternately maintained at potentials of the order of l00 volts and +100 volts with respect to ground, respectively. This is accomplished by connecting a first alternate set of rings to the positive terminal of a battery 92 and the remaining rings to an adjustable tap 94 of a potentiometer 96 which is connected across the battery 92. An intermediate tap 98 of potentiometer 96 is connected to ground in order that the aforementioned potentials may be referenoed to ground as necessary. Adjustment of tap 94 of potentiometer 96 controls the potentialgradient between successive rings and hence the ionization current between them. This method of providing ionization in the plasma region assures a substantial number of ions in the vicinity of the path of the electron stream. The resistive coating 82, in this instance is connected to ground.

The potentials applied to the annular rings 90, on the other hand, may be progressively increased as illustrative in Fig. 5. This may be accomplished by connecting the rings to a series of adjustable taps of a potentiometer 100 which is connected across a variable potential source 102, the negative terminal of which is connected to ground. In this instance the potential gradient between successive rings may be of the order of 50 volts and is varied by increasing or decreasing the volt age across the potentiometer 100. As in the case of the embodiment shown in Fig. 4, the ionization current between successive rings is in the vicinity of the path of the electron stream. A similar potential gradient is maintained along the conductive coating 82 on the inner surface of envelope 10 by connections from the end ring! to the nearest respective matching termination.

What is claimed is:

1. An electron stream device for amplifying microwave signals, said device comprising means for producing an electron stream, means for modulating said electron stream with an input signal, means for directing said modulated electron stream along a predetermined path, means for providing a plasma region contiguously along a substantial length of said path whereby the electrons of said stream interact with electric fields generated by currents induced in said plasma region by the modulations of said stream thereby to amplify said modulations, said plasma region being constituted of an ionized inert gas at a pressure of no less than 0.5 micron, and means for deriving an output signal from the amplified modulations of said electron stream.

2. An electron stream device for amplifying microwave signals, said device comprising means for producing an electron stream, means for modulating said electron stream with a microwave input signal, means for directing said modulated electron stream along a predetermined path, means for providing a region containing an inert gas at no less than 50 microns pressure along a substantial length of said path, means for controlling the ionization of said inert gas to produce interaction bctween the electrons of said stream and electric fields generated by currents induced in the ionized gas by the modulations of said stream thereby to amplify said modulations, and means for deriving a microwave output signal from the amplified modulations of said electron stream.

3. An electron stream tube for amplifying microwave signals, said tube comprising means for producing an electron stream; means for modulating said electron stream with a microwave input signal; means for directing said modulated electron stream along a predetermined path; a sealed chamber disposed contiguously along a substantial length of said path, said chamber containing an inert gas at no less than 50 microns pressure; a glass wall separating said chamber from said path; means for ionizing said inert gas to a predetermined degree whereby the electrons of said stream interact with electric fields generated by currents induced in the ionized gas by the modulations of said stream thereby to amplify said modulations; and means for deriving a microwave output signal from the amplified modulations of said ele fn stream.

4. The electron stream tube as defined in claim 3 which additionally comprises means including a resistive coating on the surface of said thin glass wall adjacent said path to prevent the accumulation of negative charge 75 on said surface.

7 5. An electron stream tube for amplifying a microwave signal, said tube comprising an envelope containing an inert garat no less than 0.5 'micron pressure;-

means within said envelope for producing an electron stream; means for modulating said electron stream with the microwave signal; means for launching said modulated electron stream along a predetermined path through the inert gas to permit said stream to ionize said inert gas and form positive ions along said path, the positive ions neutralizing the space-charge forces within said stream and thereby focusing said electron stream along said path; means for controlling the ionization of said inert gas to permit said electron stream to interact further with said inert gas and amplify the modulations of said electron stream; and means for deriving an output signal from the amplified modulations of. said electron stream.

6. The electron stream tube as defined in claim 5 which additionally includes means for providing a chamber disposed coextensive with and adjacent said path, said means including a wall of dielectric material separating said path from said chamber; an inert gas within said chamber; and means for controlling the ionization of the gas in said chamber to form a plasma region to further amplify said modulations.

7. The electron stream tube as defined in claim 5 8 wherein said means for controlling the ionization of said mert gas comprises first and second electrodes disposed adjacent to and in spaced relationship along said-path, and means for maintaining a predetermined potential difl'erence between said first and second electrodes.

8. The electron stream tube as'defined' in claim 5 wherein said means for controlling the ionization of said inert gas comprises a plurality of annular electrodes disposed concentrically about and in succession along said path, and means for maintaining alternate positive and negative potential gradients between the successive elec-.

trodes.

9.'I'heelectronstreamtubeasdefined in claim 5 wherein said means for controlling the ionization of said inert gas comprises a plurality of annular electrodesdisposed concentrically about and in uniform succession along said path, and means for maintaining predeternuned progressively increasing potentials on the successive electrodes along said path.

References Cited in the tile of this patent UNITED STATES PATENTS Dodds Mar. 3, 1953 Hollenberg Sept. 15, 1953 Hansel! July 20, 1954 

